1. Field of the Invention
This invention relates to a projection color liquid crystal display apparatus of the single panel type which projects a display image of a single color liquid crystal display element in an enlarged scale on a screen, and more particularly to a projection color liquid crystal display apparatus which is used principally to display an image in a personal computer, a television set or a video cassette recorder.
2. Description of the Related Art
Projection color liquid crystal display apparatus which project an image displayed by a liquid crystal display element in an enlarged scale are divided into the three panel type in which three liquid crystal display elements are employed and the single panel type in which a single liquid crystal display element is employed. In a projection color liquid crystal display apparatus of the three panel type, each of color lights of the three primary colors of red, green and blue obtained by color separation of light of a light source is irradiated upon one of the three liquid crystal display elements which corresponds to the color. Consequently, images are displayed by the individual liquid crystal display elements, and the images are optically composed into an image. The image composed in this manner is projected by a projection lens. However, the color liquid crystal display apparatus of the three panel type has a comparatively large size and requires a high cost because the optical system thereof is complicated. In contrast, a projection color liquid crystal display apparatus of the single panel type is superior in terms of size and production cost of apparatus to the projection color liquid crystal display apparatus of the three panel type because it involves only one liquid crystal display element.
In an ordinary color liquid crystal display apparatus of the single panel type, filters of the three primary colors of red, green and blue are formed in a a mosaic fashion in a liquid crystal display element in a similar manner as in a liquid crystal television set of the direct viewing type. When light from a light source illuminates the liquid crystal display element, color lights selected by the color filters of the individual colors pass through pixels of the liquid crystal display element. For example, for a pixel which displays red, the color filter of red is formed, and the color light of red passes through the red color filter, but the color lights of green and blue are absorbed or reflected by the red color filter. Accordingly, only one third of the light emitted from the total source inputted to the liquid crystal display element is utilized as a display image, and the projection color liquid crystal display apparatus of the single panel type has a problem in that the brightness of an image finally projected is approximately one third that of a projection color liquid crystal display apparatus of the three panel type when it employs the same light source.
One of solutions to the problem just described is disclosed in Japanese Patent Laid-Open No. 60538/1992 wherein a single liquid crystal display element provided with a micro-lens array is illuminated by light color-separated by three dichroic mirrors of special arrangement as described below.
The projection color liquid crystal display apparatus disclosed in the document mentioned above includes, as shown in FIG. 1, light source 101 including parabolic mirror 102, three dichroic mirrors 121B, 121R and 121G serving as light separators disposed in an advancing direction of light from light source 101 and inclined by different angles from each other as hereinafter described, polarization plate 108, micro-lens array 106, liquid crystal display element 107, polarization plate 109, field lens 122 and projection lens 110 successively disposed in an advancing direction of light reflected by dichroic mirrors 121B, 121R and 121G, and screen 111 to which a color display image is projected by projection lens 110. Reference symbols R, G and B in FIG. 1 represent red, green and blue, respectively. Also in the following description, R, G and B are used in the same meanings.
Dichroic mirror 121B has a characteristic that it reflects light of a wavelength region of blue and transmits therethrough light of red and green which are in longer wavelength regions. Dichroic mirror 121R has different characteristic that it reflects light of a wavelength region of red and transmits therethrough light of blue and green which are in shorter wavelength regions. Dichroic mirror 121G has still a different characteristic that it reflects light of wavelength regions of green and blue and transmits light of red which is in the longer wavelength region. The three dichroic mirrors 121B, 121R and 121G are disposed at angles successively different by several degrees from a mutually parallel condition. The angle defined by each two adjacent dichroic mirrors is designed so that color lights reflected by the dichroic mirrors may be converged to pixels corresponding to the individual color lights in liquid crystal display element 107 by micro-lens array 106.
In the conventional projection color liquid crystal display apparatus having the construction described above, white light emitted from light source 101 is converted into substantially parallel light by parabolic mirror 102 and then inputted to dichroic mirror 121B, by which the light of blue is reflected. The lights of red and green which pass through dichroic mirror 121B without being reflected are inputted to dichroic mirror 121R, by which the color light of red is reflected. Similarly, the light of green which passes through dichroic mirror 121R without being reflected is inputted to dichroic mirror 121G, by which it is reflected. The lights of the three primary colors reflected in this manner are inputted to polarization plate 108. Then, only polarized light components of a particular direction called linearly polarized light which is used for a display image pass through polarization plate 108 and then are inputted at individually different angles to micro-lens array 106. Since the color lights are reflected by three dichroic mirrors 121B, 121R and 121G inclined in such a manner as described above, the color lights are converged to pixels corresponding to the individual color lights in liquid crystal display element 107 by the individual lenses of micro-lens array 106. The color lights of R, G and B are selectively inputted to pixels of the individually corresponding colors in liquid crystal display element 107 in this manner, and emerge as a display image from liquid crystal display element 107. The emergent lights are inputted to polarization plate 109, through which only linearly polarized components pass so that they are inputted to field lens 122. The lights having passed through liquid crystal display element 107 and tending to expand are converged by field lens 122, and the thus converged lights are projected in an expanded scale to screen 111 by projection lens 110. Although field lens 122 is not particularly required, the aperture of projection lens 110 can be reduced by employing field lens 122.
As described above, with a projection color liquid crystal display apparatus which employs a micro-lens array, since color lights are selectively inputted to pixels in a liquid crystal display element which correspond to the individual color lights, no color filter is required. As a result, since the projection color liquid crystal display apparatus is free from loss of light by a color filter, it can utilize light more efficiently than another apparatus in which a color filter is employed.
With the conventional projection color liquid crystal display apparatus described above, however, while white light is separated into lights of red, green and blue using three dichroic mirrors 121B, 121R and 121G having wavelength selectivity, since light from the light source is natural light, in order to utilize the light as a display image on the liquid crystal display element, polarization plate 108 must be used to convert natural light from the light source into linearly polarized light. Accordingly, one of the polarized light components of p-polarized light and s-polarized light which are included in the natural light is absorbed and lost by polarization plate 108. In other words, the loss of approximately one half the total light emitted from the light source cannot be avoided. Further, the light absorbed by polarization plate 108 changes into heat, which gives rise to a problem that it deteriorates the performance of polarization plate 108. Consequently, there is a limitation in employment of a high-power light source.
As described above, the conventional projection color liquid crystal display apparatus is disadvantageous in that it is low in utilization efficiency of light from a light source due to absorption of light by a polarization plate and that the polarization plate is deteriorated as a result of absorption of light.
Further, fine adjustment in arrangement is required individually for the three dichroic mirrors, and besides, there is the possibility that the dichroic mirrors may be displaced out of position by vibrations.